What is the world's tallest building

It depends on who does the measuring! Skyscraper buffs disagree on whether features like flagpoles, antennas, and spires should be included when measuring building height.

Also under dispute is the question of what, exactly, constitutes a building. Technically, observation towers and communications towers are considered structures, not buildings, because they are not habitable. They do not have residential or office space.

Soaring 509 meters (1,670 feet), the Taipei 101 Tower in Taipei, Taiwan appears to be the world's tallest building. But the Taipei 101 Tower's massive 60-foot spire contributes to much of that height.

Before the Taipei 101 Tower was completed, the Petronas Twin Towers in Kuala Lumpur, Malaysia were described as the tallest buildings in the world. But, like the Taipei 101 Tower, the Petronas Towers get much of their height from spires, not from usable space.

If you count only habitable space and measure from the sidewalk level of the main entrance to the structural top of building (excluding flagpoles and spires), then Chicago's Sears Tower, built in 1974, may still be the tallest building in the world.

For awhile it was thought that New York's new World Trade Center skyscraper, Freedom Tower, would become the world's tallest building. But security concerns lead designers to scale down their plans. Freedom Tower will rise 541 meters (1,776 feet), but much of that height will be in it's needle-like spire. The structural height of Freedom Tower will be 417 meters (1,368 feet). Sears Tower in Chicago and the Jin Mao Building in Shanghai are taller. Also, even taller buildings are on the drawing board in Korea, Russia, and Dubai.

Now, supposing we included spires, flagpoles, and antennas when measuring building heights. In that case, it might not make sense to distinguish between buildings and towers when ranking building heights. If we rank all man-made structures, whether or not they contain habitable space, then the CN Tower in Canada is the world's tallest building. The communications tower and tourist attraction measures 553.33 meters (1,815 ft., 5 inches) tall.

(via About.com)